Here’s Dave Taylor’s answer:
“The big marketing debate over SEO has never been whether or not to do it. It’s self-evident that you’d want your site to appear near the top of free rankings for keywords important to your brand. Instead, the debate has always been whether to outsource or handle SEO in-house.

According to Marketing Sherpa’s brand new Search Marketing Benchmark Guide, the keep-SEO-in-house school of thought is winning. This is amply borne out when we examine the actual percent of total search marketers who run SEO in-house. Last year that percent was about 74% and this year it’s … 74%. (…)

If you’re a client-side marketer, this data points to a significant ongoing competitive opportunity for your online presence. According to our 2005 data, marketers who outsourced SEO saw a 110% lift in overall site traffic within six months, whereas marketers who handled SEO in-house saw a 73% lift. (…)

In short, if you hire an expert for SEO, you tend to get significantly better results.”